RU2504759C1 - Method for wood moisture content determination - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: method for measuring wood moisture content is proposed and implies that the sample is touched by two electrodes set along the line perpendicular to fiber grains at a fixed distance from each other. Voltage is applied to the measurement cell consisting of sequentially connected moist material and standard resistance. Drop of voltage at the standard resistance is measured and moisture content is determined. At certain time point the amplitude of voltage, current and slope of respective pulse dynamic characteristics are measured and provide for recording a set of their information parameters: time constant and limit voltage, initial current and its slope which are used to determine moisture content according to the calibration characteristic. The calibration is performed in advance on the adaptive range borders using a sample with known moisture content and standardised parameters: time constant and limit voltage, initial current and its slope when measuring at certain time point the amplitude of voltage, current and slope of respective standardised pulse dynamic characteristics.

EFFECT: improving accuracy and enhancing control range at specified metrological characteristics.

1 tbl, 6 dwg

Description

The present invention relates to measuring technique, in particular to measuring the moisture content of capillary-porous materials.

The known method [see Prince Glinkin E.I. Circuitry of analog-to-digital converters. - Tambov: Publishing house of Tamb. state tech. Un-ta, 2009. - S.62-64], which consists in measuring a voltogram, recording speed and acceleration for n response measurements, which determine the values of steady-state voltage and time constant.

The disadvantages of this method are the low metrological efficiency due to the low reproducibility of the characteristics, the influence of random disturbances from temporal, temperature and parametric drift.

The known method [see Patent No. 2240546 (RF) G01N 27/04, 2004. Bull. No. 32], which consists in registering the time of comparing the current amplitude with a threshold value and measuring the second voltage at a multiple point in time from the initial time, using the two voltages and time points, find the diffusion (limiting) current in the sample as the ratio of the amplitude of the steady-state potential to time constant by which humidity is determined.

The disadvantages of this method are the low measurement efficiency in the adaptive range due to the indirect determination of the limiting diffusion current of the sample, on which the humidity depends, as well as the methodological error due to the linearization of the current-humidity characteristics.

For the prototype adopted the method [see Patent No. 2341788 (RF) G01N 27/04, 2008. Bill. No. 35], which consists in the fact that the current amplitude of the current is recorded at the first moment of time and the second current is measured at a multiple point in time from the initial time value, the limiting current in the sample and the actual humidity are found from the two currents and time points from the calibration characteristic.

The disadvantage of the prototype is the complexity of measuring the amplitude of the current in nanoamperes, commensurate with white noise.

The technical task of the method is to increase the accuracy and expand the control range for a given metrological characteristics.

The technical task is achieved in that:

In the method for determining the moisture content of wood, which consists in contacting the sample with two electrodes located along a line perpendicular to the fibers of the sample at a fixed distance from each other, a voltage is applied to the measuring cell, consisting of series-connected wet material and a reference resistance , measure the voltage at the reference resistance and determine the humidity, in contrast to the prototype, at a fixed point in time measure the amplitude of the voltage, current and twist the corresponding impulse dynamic characteristics, according to which their complex of informative parameters is recorded: time constant and limiting voltage, initial current and its steepness, which serve to determine humidity by the calibration characteristic, calibration is carried out a priori at the boundaries of the adaptive range according to the sample with known humidity and normalized parameters : time constant and limiting voltage, initial current and slope when measuring voltage amplitudes at a fixed time, current and the steepness of the corresponding normalized pulse dynamic characteristics.

The essence of the proposed method is illustrated in figure 1 ÷ 6. The proposed method includes 2 stages:

- Measurement of the initial current of the test sample;

- Calibration on reference materials to determine actual moisture values.

1. The moisture content of wood is determined by measuring the initial current of the test sample. To do this, contact with the sample using two electrodes located along a line perpendicular to the fibers of the sample at a fixed distance from each other. Apply voltage to the measuring cell, consisting of series-connected wet material and a reference resistance R ₀ (figure 1). Wet material is presented in the form of an equivalent circuit, including the limiting voltage E, the initial current I ₀ and the dry material current I _S.

и крутизну

вольтограммы U(t) (фиг.2). Алгоритмы определения информативных параметров находят по математическому уравнению, описывающему импульсную динамическую характеристику U=U(E, T, t) дифференциальным уравнением первого порядка:At one point in time t, the voltage amplitude U, current is measured

and toughness

voltograms U (t) (figure 2). Algorithms for determining informative parameters are found by a mathematical equation that describes the impulse dynamic characteristic U = U (E, T, t) by a first-order differential equation:

where T is the time constant, E is the ultimate voltage, U is the applied voltage on the sample of material, U ′ is the speed of the voltogram U (t) corresponding to the current of the measuring cell.

Dependence (1) relates the measured value of the voltage amplitude U during the study time t to the voltage limit value E with the time constant T.

Parameters E and T uniquely determine the dynamic characteristic of the experiment according to dependence (1), therefore it is advisable to take them as informative parameters of the dynamic process of analytical control.

Formula (1) allows you to determine only one of the desired parameters, depending on the value of the other. For independent algorithms for calculating informative parameters, the second relation is found from equation (1) after differentiation in time in the form of a second-order differential equation:

.where U ′ ′ is the acceleration of the voltogram U (t) corresponding to the current slope

.

From the expression (2), an algorithm for determining the time constant T is obtained:

as the ratio of the velocity U ′ (current) to the acceleration U ′ ′ (steepness) of the voltogram U (t).

Substituting expression (3) into equation (1), the algorithm for determining the limiting voltage E is revealed:

In practice, the algorithms (3) and (4) implement in the measurement process at one moment in time t _{i the} instantaneous value of the voltage U (t), its current and slope (see figure 2).

Using the obtained informative parameters E, T and formula (1), the impulse dynamic characteristics of voltage U (t), current U ′ (t) and slope U ′ ′ (t) are restored. The obtained curves are identified by the experiment to determine the initial current, initial flow and, accordingly, the moisture content of the sample.

) находят для C=1 предельное значение начального тока влажного материала по калибровочной характеристике:2. In the proposed method using equivalents E and T by the principle of analogy (capacitive current is defined as the ratio

) find for C = 1 the limiting value of the initial current of the wet material according to the calibration characteristic:

The connection of moisture W of the test sample with the initial current I _{0 is} carried out by analogy with the I – V characteristics of semiconductors [A.24, formula (2.75), Book 1. Microelectronics. The physical basis of functioning. - M .: Higher school, 1987, p. 81] according to the gauge characteristic:

where I _S is the current structure of the dry material;

W ₀ - maximum moisture saturated with moisture W≤W ₀ material.

As for the dynamic characteristic, the parameters _IS and W ₀ uniquely determine the experimental static characteristic, therefore they are also taken as informative parameters.

From the formula (6) follows the dependence of moisture W on the initial current I ₀ :

The approximation of the experimental curve W (I ₀ ) (Fig. 3, curve 1) by the exponential dependence (Fig. 3, curve 2) does not exceed the error δ≤0.1%.

Calibration on the standards of the range boundaries serves to calculate the informative parameters W ₀ and I _S to optimize the exponential static characteristic (6) of the experimental humidity dependence.

When calibrating, the initial current values in the lower I ₀₁ and I ₀₂ are measured in the upper limits of the normalized humidity range on standard materials with known humidity W ₀₁ and W ₀₂ (Fig. 3). An algorithm for calculating informative parameters is found from a system of two equations for the first and second dimensions:

They divide the second equation of the system into the first and bring it to the form:

,

,

where n = W ₀₂ / W _0l is the coefficient of the multiplicity of humidity of the final and initial points of the range.

Expose the logarithmic equation and express the parameter I _S :

They compose a system of equations for the current I _S :

Divide the first equation of the system by the second and solve the resulting equation with respect to W ₀ :

The obtained parameters I _S and W ₀ uniquely determine the characteristics of the experiment according to dependence (6), therefore, they are taken as informative parameters and a calibration curve is constructed (Fig. 3, curve 2).

Figure 5 shows a qualitative assessment of the adequacy of the proposed method to the physics of the process: the experimental curve 1 and the curve 2 obtained by the exponential law practically coincide.

Therefore, the determination of humidity at the boundaries of the adaptive range is adequate.

Let us prove the effectiveness of the proposed method for determining humidity at a fixed point in time by the dynamic characteristics of voltage, current and steepness (Fig. 5 ÷ 6) and by the calibration characteristic of humidity at the boundaries of the adaptive range (Fig. 3 ÷ 4).

To prove the adequacy of the proposed method to the experiment, we compare the dynamic characteristics of the current U ′ (t) = I _e (t) with the dynamic current characteristics of the prototype I _i (t) (Fig. 5). A qualitative assessment is visible from the graph: the curves almost coincide. Quantitatively, the relative deviation ε of the dynamic current characteristic I _e from the experimental dynamic current characteristic I _i is determined by the formula:

where I _i is the experimental current value of the prototype, measured by the exponential dependence;

I _e - the value of the current impulse dynamic characteristics of the proposed method, recorded by the differential relationship.

For the efficiency of measurement with a given accuracy ε _{0 a} fixed point in time t ₁ measurements choose less than the time constant T <0.1. From the graph (Fig.6) it can be seen that in this interval the error is constant and does not exceed a predetermined value ε ₀ = 0.015 · 10 ^-13 %.

Therefore, the proposed method for recording voltage, current and steepness at a fixed point in time is adequate with a given error.

- число эталонов с известными влажностью W_0k и током I_ok:To assess the effectiveness of the proposed method for determining humidity by the calibration characteristic (Fig. 3) of humidity, the error δ is fixed (see Fig. 4) of the proposed method and taken for an equivalent measure of δ _0k = δ ₀ , where

- the number of standards with known humidity W _0k and current I _ok :

For each k- th error δ _0k find the ranges of the proposed method and prototype. The error of the proposed method tends to zero, for example, to 0.01, and is constant over the entire investigated range of humidity D, so the humidity range of the proposed method D ₁ is equal to the entire studied range of humidity D. The efficiency is determined by formula (12) as the ratio of the humidity range of the proposed method D ₁ to the humidity range of the prototype D ₂ . The data are summarized in table 1 and reflected in figure 4.

Table 1 δ _0k ,% D ₁ D ₂ η 0.01 0.16 0.008 twenty 0.05 0.16 0.009 17.8 0.1 0.16 0,038 1.31

The table shows that with a decrease in the evaluation criterion δ, the moisture range of the prototype D ₂ decreases. For example, when δ ₀₃ = 0.1%, D ₂ = 0.122. With a decrease in the error δ, the range D ₂ decreases, and the efficiency η of the proposed method increases by an order of magnitude. For example, for the error δ ₀₁ = 0.01%, the range is D ₂ = 0.008, and the efficiency is η = 20.

Therefore, the proposed method is effective in expanding the humidity range for a given measurement error by an order of magnitude.

Similar results are obvious for a fixed range, for which the efficiency in accuracy is also higher by an order of magnitude.

Thus, the determination in the adaptive humidity range of the initial current recorded at one fixed point in time by the dynamic characteristics of voltage, current and steepness, in contrast to the prototype, increases the accuracy and range by an order of magnitude.

Claims (1)

The method of determining the moisture content of wood, which consists in contacting the sample using two electrodes located along a line perpendicular to the fibers of the sample at a fixed distance from each other, applying a voltage to the measuring cell, consisting of series-connected wet material and a reference resistance, measure the voltage at the reference resistance and determine the humidity, characterized in that at a fixed point in time measure the amplitude of the voltage, current and slope with The corresponding impulse dynamic characteristics, according to which their complex of informative parameters are recorded: time constant and limiting voltage, initial current and its slope, which serve to determine humidity by the calibration characteristic, are calibrated a priori at the boundaries of the adaptive range according to the sample with known humidity and normalized parameters: time constant and limiting voltage, initial current and slope when measuring at a fixed point in time the amplitudes of voltage, current and utizny respective normalized pulse dynamic characteristics.

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